It is common to model human diseases in non-human mammals by modifying or deleting (excising) the specific gene or genes hypothesized to be responsible for the disease.
A commonly used technique is to remove the entire gene or an essential part of it in the animal model. There are at least two ways to achieve this. First, the gene can be removed from the germline stage, in early life, which is also called “knockout”. In the knockout animal, every cell carries the gene deletion. As many genes are essential to embryonic development, embryonic death can occur.
To solve this problem, a second technique was developed, called conditional knockout, in which a specific gene can be deleted at a specific tissue and time rather than early in life. This is commonly done by activating the transcription of a certain recombinase, such as Cre. The recombinase will delete the sequence between two recombination sites when the sites are facing the same direction. Since the expression of the recombinase is controlled by its own gene promoter, the deletion of the target gene will be determined by where and when this promoter becomes active.
The commonly used promoters are not well defined in terms of where (e.g. in which tissue) and when (e.g. developmental stage or presence of physiological stressors) they will drive the expression of the recombinase. When researchers went back to trace where and when the gene was deleted, they faced daunting problems. As organs consist of many different cells and cell types, without a reporter, researchers could not pin-point when and where the deletion had taken place.
Therefore, there is a need to monitor and visualize where and when a gene is deleted in a conditional knockout model.